Why do I need topology?
A topology is an elementary thing of everything, you can animate high poly sculpt, it would be a pain and your computer would just explode. Good topology makes model deform in a good way with low effort. 
It doesn't matter if you start with low poly from scratch or model a high poly and then do a retopology. As long as you want to manipulate the model and animate it you need the low poly. If the model is just for printing or visual it doesn't matter that much, but even tho good topology makes things easier.